For most people, waste management involves mostly taking the garbage out of their home where it is picked up by a municipal service and taken to an authorized large-scale waste processing facility. However, many small communities live in isolation, as far as waste management is concerned (either due to geographic location or because of the nature of waste generated), and must treat their waste locally. Small isolated communities can be found anywhere and may include military and commercial ships, research and military outposts, northern communities, small islands, resorts, clinics and hospitals, laboratories, industries and many others.
A typical example of an isolated community is a ship at sea (for example a cargo ship or a navy vessel). Sailors on board (of which there may be several dozen or several hundred) typically generate around 1 kilogram of waste per sailor per day and are out at sea for weeks at a time. These ships must choose between throwing their waste into the sea, storing it on board, or treating it on board. All of these options have serious problems associated with them.
The disposal of waste into the sea is restricted and the disposal of some waste, like plastic, is forbidden everywhere. On-board storage of waste can cause serious health problems, infestations by rodents, may require large refrigerated containers, and many ports will not accept the waste or will charge high commissions for the final disposal. Furthermore ships may be operating in coastlines of hostile countries, making the possible transfer of the waste to a port facility impossible. Existing methods for on-board disposal (primarily incineration) are incapable of disposing of the waste without producing harmful or toxic emissions (such as dioxins arising from the combustion of plastics).
A similar situation exists for isolated northern habitats with permafrost conditions. Landfill as a method of disposal is unacceptable (lack of biodegradation), shipping of the waste for treatment elsewhere involves exorbitant costs, and incineration in pits or small systems generates toxic emissions. Many islands face similar problems; land is at a premium and therefore landfilling as well as shipment of the waste elsewhere are not attractive options.
Generally, people pay to have their waste taken away in a manner that is usually controlled by the local government. Because this practice is so prevalent throughout the world, nearly all of the technologies that are either used commercially or under development are aimed at processing large quantities of waste in a manner that is economically feasible and environmentally acceptable. Thus, there are very few small-scale options available today for treatment of waste in isolated habitats. The systems that are available are either not compact and/or not environmentally safe, and require the use of significant amounts of externally provided energy. Available approaches for waste disposal in isolated human habitats can be separated into two categories: those that change the physical form of the waste and those that modify the chemical form.
The approaches that modify only the physical form include technologies such as compactors, shredders, pulpers and plastic processors. Generally, these approaches aim to reduce the volume of the waste, or prepare the waste so that it can be disposed of at a later date or in a different location (for example in certain approved areas of the ocean). As well, these approaches may be used to isolate problematic portions of the waste (for example plastic which cannot be disposed of at sea) until it can be properly disposed of. These approaches bear no relation to the present invention from a technological point of view; they are discussed here only because they represent some of the few waste treatment options available to isolated communities.
The invention disclosed herein is related to other technologies aimed at modifying the chemical form of the waste and specifically those causing the thermal oxidation of all organic waste. Pyrolysis, gasification and incineration (or combustion) are the main thermal processes used to modify the chemical form of waste; they are well known and used widely around the world. There are numerous patents describing all manners of operating these processes (usually on a continuous basis rather than batch operation) to improve their efficiency, reduce their environmental impact, process different types of waste, etc.
Pyrolysis operates in the complete absence of oxygen and thermally decomposes the organic waste into a carbon based char and a mixture of oils. It is not a process that is suitable for small scale waste treatment. Neither the char nor the mixture of oils can be used by a small isolated community. As such, pyrolysis is a viable process only for relatively large scale operations and for the treatment of specialty waste such as scrap tires and plastics.
An early example of a pyrolysis processes which is used for the treatment of waste to produce methane gas is disclosed in U.S. Pat. No. 4,152,122 (Apparatus for the production of methane containing gas by hydrogasification) was published in 1979. It describes various efforts to improve the energy efficiency of the process by using the sensible heat of the gas produced by pyrolysis and gasification to dry the waste. While this patent is not related to the technology disclosed herein, it shows early efforts to manage the energy balance of a thermal waste treatment furnace.
In 1982, U.S. Pat. No. 4,308,807 (Apparatus for pyrolysis of municipal waste utilizing heat recovery) describes similar efforts to improve the energy efficiency of the process by using the energy contained within the waste. This patent discloses a pyrolysis process in which some of the hydrocarbon gases produced by the pyrolysis of waste are combusted as the fuel and used to operate the pyrolysis reactor. This is somewhat similar to the idea being disclosed herein, in that it uses the fuel produced by the process to drive the process. However, in this patent, which does not target small scale applications, only a small fraction of the energy in the waste is used within the process and there is nothing similar to the small batch gasification reactor being disclosed by the present invention.
Another pyrolysis reactor was described in 1999, in U.S. Pat. No. 5,868,085 (Pyrolytic waste treatment system). This patent also discloses efforts to recover some of the heat in the process by introducing fully combusted gases that were initially used to heat the pyrolysis furnace inside the pyrolysis furnace to improve energy transfer and overall efficiency. Again, there is no effort to create a small scale system and there is no heat management system such as the one being disclosed by the present invention.
Another effort to improve the efficiency of a pyrolysis reactor used for the treatment of waste is disclosed in 2000, in U.S. Pat. No. 6,084,147 (Pyrolytic decomposition of organic waste). In this patent, a novel pyrolysis reactor is described which uses pre-heated amorphous alumina beads to heat-up and agitate the waste, and thus increase the rate of treatment. This is typical of the approaches which are commonly used to increase the efficiency of large scale thermal treatment technologies. Invariably, the system's complexity is increased in an effort to improve processing rate and energy efficiency. This is opposite to what is described in the present disclosure which discloses a very simple technology without emphasis on processing rate.
Generally, there has not been any disclosure of a pyrolysis furnace that has been designed specifically for the treatment of small amounts of waste. More typical for small scale pyrolysis would be the one described in US Patent Application Publication No: US2003/0199718A1 (Process for Converting Waste Plastic into Lubrication Oils) in which the process is more focused on the production of a specific product (such as lubrication oils in this case or carbon black in the pyrolysis of scrap tires) and less on the elimination of waste. No pyrolytic reactor that is similar to the MAGS reactor has ever been disclosed.
Gasification is a process in which a controlled amount of oxygen is used to convert the organic molecules in the waste into a synthesis gas containing mostly carbon monoxide and hydrogen. Gasification has been practiced for more than 200 years and a number of attempts have been made to develop an efficient and maintenance free gasification reactor.
As early as 1988, U.S. Pat. No. 4,764,185 (Gasifier apparatus) describes a gasifier that is designed for improved efficiency and reduced maintenance.
In 1989, U.S. Pat. No. 4,828,581 (Low inlet gas velocity high throughput biomass gasifier) describes a high throughput gasification process that uses inert sand to improve energy efficiency. The hot sand and a certain amount of air are used to convert the waste into a synthesis gas and a certain amount of char. Subsequently, the sand and the char are moved to a separate chamber and the char is burned to heat up the sand.
In 1995, U.S. Pat. No. 5,423,891 (Method for direct gasification of solid waste materials) also discloses a gasification reactor that uses pre-heated solids to improve the gasification rate.
While most technologies focused on means of increasing the processing rate of the gasification process, some also focused on producing a clean synthesis gas that could be used commercially. One such technology is described in 1995 in U.S. Pat. No. 5,470,361, in which methods are disclosed for removing HCl, sulfur compounds and dust from the synthesis gas.
In 1996, another disclosure, U.S. Pat. No. 5,534,659 (Apparatus and method for heating hazardous waste) describes a gasification furnace that is designed for higher processing rate and better environmental performance. The reactor described in this patent uses a plasma torch to heat the waste to very high temperatures and to melt the inorganic fraction of the waste into slag. This technology is perhaps the natural conclusion of various efforts to improve gasification by making a reactor that offers high processing rates and complete treatment.
Also in 1996, U.S. Pat. No. 5,553,554 (Waste disposal and energy recovery system and method) describes a process in which waste is converted into synthesis gas inside a rotary kiln gasifier. The reactor design may be different than that used in other processes, but this disclosure is typical of commercial gasification technologies. The synthesis gas produced is burned in a separate vessel to recover its energy content and to produce either steam or hot water.
An innovative thermal treatment reactor for waste is described in 1998 in U.S. Pat. No. 5,770,017 (Method for ablative heat transfer). In this reactor, the solid waste is introduced into a helically shaped vessel and conveyed through the vessel at a velocity that maintains contact between the waste and the wall. This significantly improves heat transfer to the waste and thus increases the gasification rate of the waste.
Generally, all technologies described to date on the gasification of waste are similar to the one described in 2000 in U.S. Pat. No. 6,032,467 (Method and apparatus for recovering energy from wastes). In this patent, a gasification process is described that uses a fluidized-bed reactor and a melting furnace to convert the waste into a synthesis gas. The synthesis gas is then cleaned and used to produce electricity. While the type of gasification reactor may be different from process to process, all gasification technologies disclosed to date are designed for large processing rates and the use of synthesis gas for the generation of energy, most often, electricity.
In 2003, U.S. Pat. No. 6,613,111 (Small scale high throughput biomass gasification system and method) describes a technology that uses the energy produced by the combustion of a synthesis gas to gasify waste and produce more synthesis gas. The gasification and combustion chambers described in U.S. Pat. No. 6,613,111 are concentric and the heat from the combustor is used to provide the energy needed for gasification. In the technology described in U.S. Pat. No. 6,613,111, sand is used as the heat transfer medium. Sand is heated up in the combustion zone and then transferred to the gasifier to heat up the waste. Both the combustor and the gasifier are fluidized bed reactors designed for high processing rates. In fact, the waste processing rate for this system is given at 500 to 4400 lbs/hr per square foot of the gasification reactor's diameter.
Another innovation related to gasification is disclosed in 2004 in U.S. Pat. No. 6,790,383 (Method of gasifying carbonaceous materials). While the system described in this patent is used for the processing of coal, shredded tires and waste oils, it may be adaptable to be used with municipal solid waste. In this disclosure, a process is described which uses some of the synthesis gas produced by gasification to provide energy for the gasification process. Specifically, some of the carbon monoxide and hydrogen in the synthesis gas are recycled into the gasification furnace where they are combusted fully to carbon dioxide and water. The energy released from the combustion of the synthesis gas is used by the gasification process. This disclosure, similarly to U.S. Pat. No. 6,613,111, teaches that the energy content of the synthesis gas can be used to convert waste into more synthesis gas.
To date, nearly all efforts to design a very small waste treatment system have been based on incineration. In incineration, the organic waste is mixed with excess air and a combustible fuel. Consequently, the waste is completely burned and all carbon is converted to carbon dioxide. In large conventional incinerators, the hot exhaust gas is used first to recover energy and then cleaned prior to being released into the environment. Incineration is the main thermal treatment technology used commercially for the elimination of waste and the recovery of energy from the waste.
There have been a number of efforts to design a very small scale incinerator, including some that have been successfully commercialized. For example, many ships use small incinerators to treat solid waste. Additionally, small incinerators have been used by many farms to dispose of animal carcass waste and hospitals for the disposal of biomedical waste. Primarily, small incinerators use conventional technologies and many are “home-made”. In fact, in many parts of the world, from Alaska to Africa governments provide instructions to citizens on how to build small incinerators for local use.
The 1997 U.S. Pat. No. 5,619,935 (Portable incinerator heat recovery device and method of use) describes a modification to a conventional small (portable) incinerator. The innovation described in this patent relates to a device designed to recover some heat from the combustion of waste. Another technology described in 1999, in U.S. Pat. No. 5,941,184 (Controlled thermal oxidation for organic waste), relates to methods for minimizing the polluting emissions from small scale incinerators. In this 1999 patent, the direction of the air flow through the incinerator is controlled and a secondary combustion stage is used to reduce emissions.
All small-scale incinerators are characterized by numerous operating and environmental problems which make them not suitable for eliminating the waste of small isolated communities. In fact, in the study completed for the World Health Organisation (WHO) by Batterman, S. entitled “Assessment of small-scale incinerators for health care waste”, from the Department of Environmental Health Sciences, University of Michigan, published in January 2004, it is concluded that no small-scale incinerator available today meets current environmental protection regulations. Awareness of the inadequacy of small incinerators is growing; for example the Minnesota Pollution Control Agency has placed a ban on the use of small, poorly controlled and operated incinerators, which are estimated to be responsible for 93% of the dioxin emission from waste combustors in Minnesota. See “Facts about the ban on small, on-site incinerators”, AQ Doc. #4.10, May 1998 by the Minnesota Pollution Control Agency, St. Paul, Minn.
There is information found in literature describing efforts to increase the efficiency of incinerators by using the heat by-products of the same incinerators. However, in 2005, U.S. Pat. No. 6,962,117 (Method and apparatus for controlling combustion in a furnace) describes an incinerator which the flue gas generated by the burning in the combustion zone is recirculated back to the combustion zone in order to reduce the temperature in the combustion zone and avoid the melting of the ash.